Our Changing Climate

Increasing U.S. Temperatures

Annual average temperature over the contiguous United States has increased by 1.2ºF (0.7°C) over the last few decades and by 1.8°F (1°C) relative to the beginning of the last century. Additional increases in annual average temperature of about 2.5°F (1.4°C) are expected over the next few decades regardless of future emissions, and increases ranging from 3°F to 12°F (1.6°–6.6°C) are expected by the end of century, depending on whether the world follows a higher or lower future scenario, with proportionally greater changes in high temperature extremes.

Over the contiguous United States, annual average temperature has increased by 1.2°F (0.7°C) for the period 1986–2016 relative to 1901–1960, and by 1.8°F (1.0°C) when calculated using a linear trend for the entire period of record.85 Surface and satellite data both show accelerated warming from 1979 to 2016, and paleoclimate records of temperatures over the United States show that recent decades are the warmest in at least the past 1,500 years.86

At the regional scale, each National Climate Assessment (NCA) region experienced an overall warming between 1901–1960 and 1986–2016 (Figure 2.4). The largest changes were in the western half of the United States, where average temperature increased by more than 1.5°F (0.8°C) in Alaska, the Northwest, the Southwest, and also in the Northern Great Plains. Over the entire period of record, the Southeast has had the least warming due to a combination of natural variations and human influences;87 since the early 1960s, however, the Southeast has been warming at an accelerated rate.88,89

Figure 2.4: Observed and Projected Changes in Annual Average Temperature

This figure shows five maps of the United States. The first map shows observed changes in annual average temperature, with increases in all regions save for some pockets of decreases in temperature in the Southern Great Plains and Southwest. The next two maps show changes in temperature by the mid-21st century under a lower scenario (RCP4.5) and higher scenario (RCP8.5). These show increases, particularly at higher latitudes, of up to 3 degrees Fahrenheit (lower scenario) and 4 degrees Fahrenheit (higher scenario) in the contiguous United States and even higher in Alaska. The last two maps show changes in temperature by the late 21st century under a lower scenario (RCP4.5) and higher scenario (RCP8.5). These increases are greater than in mid-century, with all states showing increases of 3 to 8 degrees under the lower scenario and 6 to 8 degrees Fahrenheit under the higher scenario.

Figure 2.4: Annual average temperatures across North America are projected to increase, with proportionally greater changes at higher as compared to lower latitudes, and under a higher scenario (RCP8.5, right) as compared to a lower one (RCP4.5, left). This figure compares (top) observed change for 1986–2016 (relative to 1901–1960 for the contiguous United States and 1925–1960 for Alaska, Hawai‘i, Puerto Rico, and the U.S. Virgin Islands) with projected differences in annual average temperature for mid-century (2036–2065, middle) and end-of-century (2070–2099, bottom) relative to the near-present (1986–2015). Source: adapted from Vose et al. 2017.85

Over the past two decades, the number of high temperature records recorded in the United States far exceeds the number of low temperature records. The length of the frost-free season, from the last freeze in spring to the first freeze of autumn, has increased for all regions since the early 1900s.85,90 The frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s. Over timescales shorter than a decade, the 1930s Dust Bowl remains the peak period for extreme heat in the United States for a variety of reasons, including exceptionally dry springs coupled with poor land management practices during that era.85,91,92,93

Over the next few decades, annual average temperature over the contiguous United States is projected to increase by about 2.2°F (1.2°C) relative to 1986–2015, regardless of future scenario. As a result, recent record-setting hot years are projected to become common in the near future for the United States. Much larger increases are projected by late century: 2.3°–6.7°F (1.3°–3.7°C) under a lower scenario (RCP4.5) and 5.4°–11.0°F (3.0°–6.1°C) under a higher scenario (RCP8.5) relative to 1986–2015 (Figure 2.4).85

Extreme high temperatures are projected to increase even more than average temperatures. Cold waves are projected to become less intense and heat waves more intense. The number of days below freezing is projected to decline, while the number of days above 90°F is projected to rise.85